In the mass production stage of the manufacture of semiconductor devices, an optical stepper having high-productivity has been used. In the production of memory devices, for example, 4-G DRAMs and higher-capacity memory devices, having a linear width of 0.1 μm or less, an electron beam exposure method having high resolution and high productivity has been attracting attention as one of exposure techniques that replace an optical exposure method.
A conventional electron beam exposure method includes a single-beam Gaussian method and a variable forming method.
FIG. 9A shows the arrangement of an exposure apparatus that employs the conventional electron beam exposure method. An electron beam EB emitted from the top surface of a cathode electrode 201 passes through an aperture 202a of a bias electrode 202, forms a crossover CO between the bias electrode 202 and an anode electrode 203, passes through an anode electrode 203 and an aperture 204a integrally formed with the anode electrode 203 to become incident on the first and second illumination lenses 205a and 205b, and passes through an aperture 204b to become incident on a projection lens 207. After that, the electron beam EB is deflected by a deflector 208 and reaches a wafer 210. Reference numeral 211 denotes a stage which places the wafer 210 thereon and moves it.
In recent years, as a method of improving the productivity of the electron beam exposure method, a cell projection method has been proposed (e.g., see Japanese Patent Laid-Open No. 2000-331632). According to this method, the repeated circuit pattern portion of a memory device or the like is divided into several-μm regions, and the entire divided pattern regions are exposed at once. This method can improve the productivity. The line width accuracy is as important as the productivity. In order to ensure the line width accuracy, the irradiation strengths of the exposure regions must be uniform with a small difference of 1% or less throughout the entire exposure regions.
The area that can be exposed at once with the cell projection method is approximately 5 μm2. The converging half-angle of the projection lens is set to several mrad in accordance with the resolution condition due to the lens aberration. Hence, as a condition required for uniform illumination, emittance ε defined by the product of the crossover diameter of the electron gun and the extracting half-angle of the irradiation beam must satisfy ε>(exposure area×converging half-angle) (=˜10 μm·mrad).
Regarding the type of the electron gun, an electron gun having a three-pole electron gun structure, which emits an electron beam with energy of about 50 kV, is generally used. To obtain a highly uniform beam from the emission electron beams emitted from the electron gun, a beam within a range of several mrad that provides good characteristics is selected from the emission electron beams emitted within an angular range of several ten mrad, and is used as the irradiation beam (for example, as shown in FIG. 9B, this beam is selected by using the aperture 204a).
This electron beam exposure apparatus uses a three-pole-structure electron gun which uses single-crystal boron hexafluoride (LaB6) to form a cathode electrode. The emission current of the cathode electrode is 100 μA to 200 μA, and several μA are extracted from the beam current and are used as an electron beam that contributes to exposure. Hence, most of the emission current is shielded by a shielding electrode portion on the way. In the example of the conventional electron gun, as the total energy of the electron beam when the acceleration voltage is 50 kV is comparatively as small as 5 mW to 10 mW, substantially no heat is generated by shielding the electron beam. Therefore, most of the energy of the electron beam is dissipated in the column, and forced cooling is not accordingly performed.
As shown in FIG. 8, when brightness of 1.0 to 1.5×106 A/cm2sr and emittance of 15 to 30 rad μm·mrad are to be obtained as the characteristics of the electron gun that realize a high throughput, the current amount of the electron gun is derived by current=(brightness)×(emittance)2. An emission current of about 10 times or more that of the conventional electron gun is required.
In the large-current-type electron gun satisfying the conditions of high brightness and large emittance in this manner, when the emission electrons generated by the cathode electrode 201 of the electron gun are shielded by the aperture 204a as in the conventional electron gun shown in FIG. 9A, the aperture 204a may dissolve. When the electrons are shielded by the anode electrode 203, the electrode portion may dissolve. When the emission electrons from the cathode electrode 201 do not irradiate the anode electrode 203 but are shielded in the column of the exposure apparatus, the scattering electrons from the shielding portion cause a charge-up phenomenon in the column. This degrades the position accuracy of the electron beam.
The electron gun is used with a high voltage of 50 kV or more. Therefore, the scattering electrons and secondary electrons generated when the emission electrons irradiate the anode electrode 203 scatter in the acceleration space of the electron gun. This causes weak discharge.
Therefore, it is difficult to realize a high-throughput electron beam exposure apparatus having a large-current-type electron gun that satisfies high brightness and large emittance.